If you’re ever at a beach with some black sand in the mix, run a magnet through the sand to see if the black sand sticks. Exploratorium educator Ken Finn demonstrates this experiment with a magnet and a plastic cup at San Francisco’s Ocean Beach. It’s how he collects grains of iron oxide and magnetite for an Exploratorium exhibit about magnetism and the magnetic lines of force. From SFGate:
The Ocean Beach black sand is unlike the variety found on Hawaiian beaches as volcanic sand is not metallic… Ocean Beach sand comes from the Sierra Nevada where igneous rocks are being worn down through erosion.
“Most of the sand at Ocean Beach is from this source,” Finn says, “carried here by the tributaries of the San Joaquin and Sacramento rivers that meet in the delta and flow through San Francisco Bay and out the gate. The prevailing ocean current helps push this exiting sediment flow south and wave action pushes it onto Ocean Beach.
The black sand is always coming and going, depending on storms, currents, the waves, the tides, the movement of the ocean.
Update: We had fun doing this at Fort Funston just south of Ocean Beach:
If you collect any magnetic sand, find a plastic bottle, a cylindrical magnet, a test tube, a funnel, and some masking tape to try this DIY science project next: Magnetic Lines of Force.make a DIY squeeze box, Liquid Sand Hot Tub, Ferrofluid + Glow Sticks, and Chladni Plate: Sand Vibration Patterns.